Expansion of Electromagnetic waves

In summary: The energy carried by the waves decreases according to an inverse square law as the amplitude of the waves increases. But wouldn't the amplitude also drop off according to the square of the distance, since the waves are spreading out?In summary, the electromagnetic waves expand at the speed of light due to the inverse square law of the electric and magnetic fields.
  • #1
Feynmanfan
129
0
Hi there Physics masters!

How on Earth do electromagnetic waves expand at the speed of light to infinity (in vacuum) if we've learned that the electric and magnetic fields vary inversely proportional to the square of the distance?

I don't know if it's a stupid question but I'm quite baffled about it.

Thanks!
 
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  • #2
I do not understand what your problem is? Could you please elaborate?
 
  • #3
Certainly.

Well, as in my first electromagnetism course I've learned that the electric field and magnetic field are both inversely propotional to the square of the distance.

Now, electromagnetic waves can expand infinetely in space and can carry ENERGy (that's what I felt today at the beach, the sun was burning!). SO my question is, how is it that these waves can carry energy and can expand to infinity in vacuum if the fields that create them vary inversely proportional to the square of the distance.

I'm a first year physics student so if you think this question has no sense just tell me and I'll think about something else.
 
  • #4
Feynmanfan said:
How on Earth do electromagnetic waves expand at the speed of light to infinity (in vacuum) if we've learned that the electric and magnetic fields vary inversely proportional to the square of the distance?

Electrostatic fields of a point charge are inverse-square.
This is one configuration of the electromagnetic field.

A plane electromagnetic wave is a different configuration of the electromagnetic field.
This configuration involves a dynamical electric field and a dynamical magnetic field, which sustain each other and propagate disturbances at the speed of light.
 
  • #5
Feynmanfan said:
Now, electromagnetic waves can expand infinetely in space and can carry ENERGy (that's what I felt today at the beach, the sun was burning!). SO my question is, how is it that these waves can carry energy and can expand to infinity in vacuum if the fields that create them vary inversely proportional to the square of the distance.
I'm a little puzzled that you are puzzled! :wink:

It is certainly possible for EM waves to be emitted from a "point" source and expand outward, carrying energy. Since the wave covers the entire area of an expanding sphere, wouldn't you think that its energy is also spread out in an inverse square pattern?

Note that it's the energy per unit area that drops off according to an inverse square law, not the field itself. The energy per unit area carried by the field is reflected by the size of the field squared--so the amplitude of the field actually drops off linearly. Does that make some sense?
 
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  • #6
Thanks.

Now I understand
 

1. What is the expansion of electromagnetic waves?

The expansion of electromagnetic waves refers to the phenomenon in which electromagnetic waves spread out as they travel through space. This is due to the property of wave propagation, where the wavefront expands as it moves away from its source.

2. How do electromagnetic waves expand?

Electromagnetic waves expand due to the principle of superposition, which states that when multiple waves are present in the same medium, they combine to form a resulting wave. As the waves travel through space, they interfere with each other and cause the wavefront to expand.

3. What factors affect the expansion of electromagnetic waves?

The expansion of electromagnetic waves can be affected by several factors, such as the wavelength of the wave, the medium through which it is traveling, and any obstacles or barriers in its path. The speed of the wave also plays a role in its expansion, as longer wavelengths tend to travel faster and therefore expand more quickly.

4. How does the expansion of electromagnetic waves affect communication?

The expansion of electromagnetic waves can impact communication in various ways. For example, in wireless communication, the expansion of the waves can cause them to weaken and lose their strength, resulting in a weaker signal. This can lead to interference and disruptions in communication. However, technological advancements such as signal amplification and directional antennas help mitigate the effects of wave expansion.

5. Is there a limit to the expansion of electromagnetic waves?

There is no limit to the expansion of electromagnetic waves. As they travel through space, their wavefront continues to expand, but the rate of expansion decreases as the distance from the source increases. However, the expansion of the wavefront will never completely stop, as long as the wave is traveling through a medium with no barriers or obstacles.

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